Explorations with Ground Water

PART I - Computer Modeling

Groundwater is "out of sight, out of mind".  But what happens when it becomes a problem?

The story is a simple one.

• The topography is gentle, sloping from about 18 m above datum in the north to the river at about 14 m in the south.
• The geology is stratigraphically simple with an upper aquifer, a middle aquitard, and a lower aquifer.
• There is an airport. The airport has a refueling area and fuel has apparently leaked in this area. There are a couple of municipal water supply wells in the southeast corner, each pumping 200 m³/day, and a river along the southern boundary.  The river is not contaminated, but the municipal wells are beginning to show traces of jet fuel!
• The supply wells are screened (open to the ground water) below the aquitard, but they are contaminated by fuel. How is this possible?
  • A hydrogeologist has suggested that an abandoned borehole south of the airport may have an influence.
• A model for the area has been created to test the possibility that the bore hole is responsible. You are working with one model and three related scenarios.
  • The model has 6 layers. Each is 2.5 m thick. The bottom two represent the lower aquifer. The middle two are the aquitard, and the upper two (the top layer thicker and extending to ground level) are the upper aquifer.
  • The first scenario is the model with only the supply wells. If the borehole was not present, could the supply wells be contaminated?
  • The second is a model that has the abandoned borehole and supply wells in it. Is it possible that the abandoned borehole could contribute to contamination of the supply wells?
  • The third adds one or more pumping wells which could be used to remove contaminated water. The water would be skimmed, oxygenated, charcoal-filtered and dumped into a sewer drain with the permission of the local city engineer. Will the removal action solve the problem? Can you successfully install one or more wells to prevent the pollution from getting to the local stream or water supply?

We will run this model three times.

• One run is no wells except the supply wells. (DEMONSTRATION)
• One run is with an abandoned well. (DEMONSTRATION)
• YOU will run the model the third time, with a pumping well to collect the contaminated groundwater.

How to run the models.

Invoke Visual Modflow from the Desktop icon.

1)  Use "File", "Open" to access "Desktop\Modflow Scenarios\No Well\es102n.vmf".

• The model has already been configured to recognize the topography, recharge by rainfall and contamination, and other critical variables.  [It took about an hour!]  We have already run the model (see below) - now, we will only 
  1. View the "Output". Describe the contour map on the pressure surface - the water table of the upper aquifer.
  2. View "Pathlines".  Describe them.
  3. Use "View Columns" to place a pink selected column (N-S) on the map and look at a cross-section of the movement of contaminated particles.  Describe and explain it.
  4. Use "View Layer" to see the potentiometric surface contours in the lower aquifer, and compare it to 1. (above).   Describe and explain the differences.

2)  Use "File", "Open" to access "Desktop\Modflow Scenarios\Abandoned Well\es102a.vmf".

• Use "Save as" to save the model to the "C:\Windows\temp" directory under the name "junk".
• The model has already been configured to recognize the topography, recharge by rainfall and contamination, and other critical variables.
• Select "Run"
  • Run  the model for "steady-state" conditions.
  • Invoke "Translate\Run" to run the model, selecting "Modflow" and "Modpath".
  • Again, "Translate & Run".
  • Do not press anything as the machine compiles the model and runs, first Modflow, them Modpath.  It will take 50-100 iterations.
  • When done (blue checks next to icons), "Exit" the Run mode.
  • View "Output" and "Pathlines".   Describe them.
  • Use "View Columns" to look at a cross-section of the movement of contaminated particles.  Describe and explain it.  [Click on "View Layer" and select uppermost unit to return to map view.]

3)  Now it's your turn!  A logical form of groundwater mitigation is pumping and treating.  Can you stop the contamination from reaching the city wells or the river?  To attempt to do so - load the model, add intervention wells as desired, and run the model!  If you were  a consulting hydrogeologist, you would bid on the contract.  In your bid:

• Wells cost $2000 plus $30/m of depth.
• Wells within 300 m of the runway cost $5000 plus $60/m (because of the need to work after dark).
• Wells ON the runway are not permissible!
• Groundwater pumping and treatment costs $0.50 per cubic meter per day (assume minimum of six months) - wells cannot produce over 200 m³/day.

Now - let's see how you can do!

• Select "File", "Main Menu".
• From the main Menu, select "Input", "Wells", "Pumping Wells" to get to the well menu.  Examine the properties of one of the existing supply wells  by using "Edit well" and clicking on the well symbol.  Note that each well must have a name, a stop day (3650), a pumping rate, and a screened depth.  NOTE:  do not change these properties!
• Cancel out to the Wells screen and decide what to do.  Where should intervention wells be drilled?  How many wells can you afford to drill - at most two?  How much do you need to pump - if you pump more than a total of 200 m³/day, it will get very expensive!
• Then, using "Add well", click where you want the well, enter a well name, enter a stop day (3650) enter a pumping rate as a negative number between -200 and zero, then click on "add screen" and click and drag to define its depth.  Remember, the lower aquifer is from 0 to 5 m, the upper aquifer from 10 m to the ground surface.  You can add more than one screen (pump from more than one depth) if you wish.  Click on "OK" to accept your well.
• "Run" the new model as described above.
• View the "Output", "Pathlines", and "View Columns".  If you are successful in collecting all of the contaminants describe and explain your observed pattern and draw it on your map.. If not, modify your well or wells and rerun the model.  When successful
  • Measure the distance of your well(s) from the nearest runway - marginal scales are in meters.  Base cost is $2000 or $5000.
  • Record the depth (in meters) to the bottom of the lowest screen.  Drilling cost is $30 or $60/m.
  • Record your pump rate.  Cost is $0.50 x 180 days x rate.
  • Calculate the bid = sum of the above costs.

How well did you do?  Record the costs and bid on your answer sheet.

Note: you can use the procedure above to move the well, add a well, change the screen length, and/or change the pumping rate.  What is the cheapest bid you can make for the job?

Submit a bid to the town council from each team of two students, complete with costs, screen depth(s), pumping rates and a map of modeled flowlines.

Use Windows Explorer to go to Windows\Temp and delete all of your "junk" files.

Part II - Physical Modeling (if time permits)

Purpose:

• Explore ground water by observing flow behavior in a physical model.
• Concepts that may be examined include head, piezometers, wells, ground-water flow, Darcy's Law, ground-water velocity, ground-water contamination, gradient, recharge, discharge, artesian flow.

Equipment and materials:

• Physical model, dyes, 4-50 ml beakers, vacuum flask, two buckets, vacuum flask, hand vacuum pump, two 60 ml syringes, two pump tubes with epoxied pipette tips, two 500 ml bottles, two two-hole stoppers with glass tubes (one set of tubes short and one set of tubes long), a supply of water either in faucet or in a carboy.

Setup:

1. Fill the model with water by inverting water-filled 500 ml bottles in the recharge reservoirs. The short-tube bottle should go in the reservoir near injection wells IS, II, ID; the long-tube bottle should go in the reservoir tapped with outlet O1.
2. While the model is filling, prepare dyed water (blue, green, red) by placing 1 or two drops of vegetable dye in the 50 ml beakers and filling the beakers with water. Use the syringe with dulled needle to inject dye into the piezometers (Blue dye into the deep piezometers (PA and PE); Green Dye into the shallow piezometers (PB, PC, PD, PF). The dye helps visualization. The other dye colors will be used later.
3. As the model is filling, look at the artesian well in the river bed. Discuss what you see.

Principles:

Wells in the model, from which water may be pumped, are labeled W1 and W2. Piezometers, in which the changes in groundwater pressure are observed, are labeled PA through PF.

Some simple experiments:

1.  Use the piezometers with dye in them.

• Record the water levels in the piezometers and wells in the table below. This record establishes your undisturbed initial head values. The graph paper on the side of the model is spaced at 1 cm. Use the top line as 50. (Fifty is an arbitrary datum which avoids the need to use negative numbers.)
• Are all heads in the model the same? Discuss.
• Remove the recharge bottles.
• What is the difference between the wells and piezometers? To explore this idea pump the well and pump a piezometer. What do you notice about the pumping rates? Reinject the appropriate dye into the piezometer you pump when this experiment is finished.
• Notice that the recharge reservoirs have emptied. How has this affected the piezometer heads?
• There are at least two (probably more) ways to calculate discharge rates from the pumping wells. What strategies could you use? Of what value would such data be?

2.  Exploration of flow isolation due to confining layers.

• The coarse-grained and fine-grained layers are separated by a bentonite clay layer. What impact do you expect this layer to have?
• Explore your expectation. Pump well W2.
• Compare the response in piezometers PA and PE with that in the other piezometers. Discuss.
• Are the water levels in piezometer PA and PE the same? Discuss.

3. Exploration of discharge.

• Look at the river. Is there a relationship between the water level in the river and the reservoirs?
• Open outlet O1. What happens? Does the water continue to flow after a short time? At what rate? Could you design an experiment using Darcy's law to calculate the hydraulic conductivity? Consider how you would design the experiment and what problems you would anticipate.

4. Exploration of contamination.

• Refill the recharge reservoirs until they stabilize.
• Pour red-dyed water into the leaky lagoon.
• What happens?
• At what rate does the dye move?
• Pump well W1.
• What happens now?
• How fast does the plume move?
• Refill the lagoon.
• Pump well W2.
• Compare what happens with what happened when you pumped well W1.

Briefly compare and contrast the physical and computer models as tools for learning and predicting groundwater behavior.